Terrain correlation and image correlation techniques have been used for military airborne navigation for some time. In terrain correlation, a geo-referenced elevation map, such as a digital terrain elevation database (DTED), is correlated against a measurement of the ground elevation taken from an aircraft using a ranging sensor such as a radar device or a LADAR (Laser Detection and Ranging) device. The map surface and sensor generated surface are compared using a two dimensional correlation algorithm operating in the elevation domain, which generates a maximum correlation when the two surfaces are perfectly overlaid. This operation produces an estimate of the offset between the two, and because the elevation map is geo-referenced, an estimate of the error in the aircraft position. The error estimate may be used in a navigation system to correct position error using a Kalman Filter or other correction technique.
Image correlation techniques work in a similar fashion. An image taken from an aircraft is correlated in the intensity domain against a georeferenced image database. The maximum correlation occurs when the two images are perfectly overlaid. The amount which the airborne image must be shifted or rotated provides an estimate of navigation error.
There are both advantages and limitations to terrain correlation and image correlation navigation methods. Terrain correlation techniques often use DTED databases, which cover virtually the entire planet, are relatively compact in terms of data storage, and are already widely used in military applications. However, the ranging sensors required for terrain correlation are typically large, heavy, expensive, and active. In addition, as the ranging sensors emit energy in the RADAR or other bands, these sensors are easily detected by adversaries. In contrast, while image correlation can be performed with small, inexpensive cameras, geo-referenced databases have extremely large storage requirements, which are prohibitive for some applications. Further, both terrain and image correlation techniques have the disadvantage of being computationally intensive, imposing high performance requirements on the processing system.